The invention relates to a process for making a super-dry finely dispersed silicon dioxide by heating silicon dioxide, particularly such silicon dioxide as is obtained in a pyrogenic process of production.
The specific properties of finely dispersed silicon dioxide which is formed by the hydrolytic cleavage of silicon tetrachloride in a flame are largely due to the process of making the material and are particularly characterized by the small size of the primary particles, the desirable particle size distribution and the structure of its surface. These properties distinguish this type of silicon dioxide which is also referred to as a fume silica or silica aerogel from other finely dispersed silicic acids such as are produced by a wet process.
The manner of making pyrogenic silicon dioxide results in a surface structure which is formed essentially by three types of so-called surface hydroxyl groups which are simultaneously present in all silicon dioxide aerogels. These three types of hydroxyl groups are the following:
1. Silanol groups which are present at the surface and which, because of the spacing from other silanol groups and their limited action range, cannot enter into any interaction and therefore may be called isolated or "free silanol groups."
2. Silanol groups of the same type as described at (1) which, however, are so closely spaced that they can interact together by formation of hydrogen bridges and which may therefore be described as "hydrogen-bonded silanol groups," and are referred to herein simply as bonded silanol groups, and
3. the hydroxyl groups of surface-adsorbed water present in the silicic acid aerogels.
The properties of the silicon dioxide aerogels are not determined by the totality of all its hydroxyl groups nor, as can be shown experimentally, by the totality of all silanol groups; rather, each of the three different hydroxyl types has its share in the particular properties of the product.
For industrial purposes a filler is generally preferred which consists of a highly pure and dry silicon dioxide which, in addition to silanol groups, still includes hydroxyl groups. On the other hand, as adsorbent or also as a filler for specific purposes, a silicon dioxide is preferred which is as free as possible of hydroxyl groups and closely spaced silanol groups.
When making a finely dispersed silicon dioxide in the flame hydrolysis process, it is customary to effect the precipitation of the finely dispersed oxides at temperatures where undesirable condensation of the products formed during the reaction, such as water or aqueous hydrochloric acid, is avoided. The final products, however, still include certain additives depending on the type of starting products and the thermic oxidation or hydrolysis. For instance, if the process starts from silicon tetrachloride, a product is obtained which still retains certain amounts of hydrogen chloride as determined by the high adsorption properties of the formed silicic acid aerosol. In order to remove the hydrogen chloride, it is well known to subject the finely dispersed oxides to a heat treatment at temperatures of about 200.degree. to 500.degree. C. and, in any case, below red heat. This treatment may be effected immediately after precipitation. Another method is to subject the oxides to a deacidification by means of superheated steam at similar or also lower temperatures in a unidirectional current or countercurrent. In this case, a good deacidification is obtained, but a certain residual humidity cannot be eliminated from the aerogel.
One of the most important properties of the silicon dioxide aerogel is its thickening action in which the adsorbed humidity, that is adsorbed water, is important. It has been found in this connection that an anhydrous silicon dioxide aerogel has a better thickening action in carbon tetrachloride than a silicon dioxide aerogel which still contains 1 to 2% water. The products thus obtained usually meet the requirements. However, there are special uses for which absolutely dry products are necessary.
Such absolutely dry silicon dioxide aerogel can be obtained by a vacuum treatment at specific conditions such as an increased temperature or with the aid of isothermal distillation. However, the product will still retain a more or less substantial amount of "bonded silanol groups."
Scientific investigations have shown that silicon dioxide aerogel which was obtained by pyrolitic decomposition of, for instance, silicon tetrachloride to a silicon dioxide aerogel with subsequent coagulation and after pressure shaping into thin laminae, this silicon dioxide not only loses the adsorbed water while being heated to 800.degree. to 1000.degree. C. through extended periods such as 8 hours at a reduced pressure of for instance 10.sup..sup.-3 Torr, but also releases some of its silanol groups by condensation to water. This process can be traced by infrared spectroscopy since the individual hydroxyl types result in different band spectra. In this connection it is of particular interest that the so-called "free silanol groups" which are visible in the infrared spectrum as a well-defined band are retained, at least up to a certain extent, at this temperature. At less extended heating periods, the "bonded" silanol groups are likewise retained to a substantial extent since the completeness of the condensation of these groups depends both on the temperature and the time of treatment.
All the described processes used for making absolutely dry products have, however, the shortcoming that they either do not lead to products which are free of "bonded" silanol groups or, as in case of the last-mentioned investigations, cannot be used for mass production and, besides, also result in products which no longer have the loose structure of the starting product and are therefore technically useless.